Base station system, and wireless communication method

ABSTRACT

When directional transmission using user multiplexing is performed in CDMA communication, the phase offset for setting antenna balance adaptively is determined. By this means, the antenna elements giving the maximum or minimum amplitude are dispersed reliably among individual users. As a result, the amplitude bias toward a specific antenna element can be decreased and the load on the transmission amplifier is further alleviated.

TECHNICAL FIELD

The present invention relates to a base station apparatus equipped withan array antenna that has a plurality of antennas used in a digitalradio communication system, and a radio communication method.

BACKGROUND ART

In a digital radio communication system, adaptive radio transmissiontechnology is applied and an array antenna that has a plurality ofantenna elements is used. A linear array antenna is used as one kind ofarray antenna. This linear array antenna has a configuration in which aplurality of antenna elements are placed in a straight line at intervalsof half the wavelength of the carrier frequency.

With a linear array antenna, complex amplitude multiplication isperformed for received signals that are input via the respectiveantennas, to give an arbitrary directivity. This technology has beendisclosed in Unexamined Japanese Patent Publication No. 9-284200. Here,as an example of a linear array antenna, a base station apparatusequipped with a linear array antenna comprising four antennas arrangedin a straight line will be described.

FIG. 1 is a block diagram showing the configuration of a base stationapparatus equipped with a conventional linear antenna. With this basestation apparatus, four antennas 1 to 4 receive radio signals, theprescribed radio reception processing (down-conversion, A/D conversion,etc.) is performed for the respective radio signals by RF sections 5 to8 provided for each antenna, and a signal in the stipulated frequencyband or intermediate frequency band is obtained. Then, in this basestation apparatus, this signal is subjected to demodulation processingand receive data is obtained.

With regard to transmit data, on the other hand, after digitalmodulation by a modulation section 10 for the respective users, thesepost-modulation signals are linearly added, the prescribed radiotransmission processing (D/A conversion, up-conversion) is performed byRF sections 5 to 8, and the signals are transmitted to antennas 1 to 4.

When a linear antenna is used in a base station apparatus, whencommunication is performed the signal power in a specific direction isadjusted, giving arbitrary directivity. In this case, a weight vectorproduct section 9 finds the complex amplitude product for the receivedsignals from each antenna, and gives an arbitrary directivity using theresult. For example, if communication is performed with a mobile station12 in direction θ from a base station apparatus 11, as shown in FIG. 2,the product is found of the complex amplitude vector shown in expression(1) below for the received signals in the order of the antennas of thebase station apparatus 11.

W=[1,exp(−jπ sin θ),exp(−j2π sin θ),exp(−j3π sin θ)]T  expression (1)

By performing the above kind of vector multiplication for the receivedsignals, the signal transmitted from the base station apparatus 11 hasmaximum power in the θ direction, and it is possible to form a powerdistribution (beam) that becomes weaker with increasing distance from θ.With four linear array antennas, it is possible to drop the signal powerby half or more when θ±22.5 is exceeded.

The formation of this power distribution is called beam forming in the θdirection. By means of this beam forming, it is possible to increase thesignal power for the desired signal, so that in communication with acertain user there is little susceptibility to influence by receivedsignals from other user directions, and it is also possible to reducethe power of transmitted signals to users in other directions thatconstitute unwanted waves. It is possible, for example, to performsimultaneous processing as shown in FIG. 2, with beam 14 in the θdirection used for communication with mobile station 12 in the θdirection, and beam 15 in the φ direction used for communication withmobile station 13 in the φ direction.

By this means, it is possible to expect improved quality ofcommunication with users, and increased communication capacity of thesystem as a whole.

However, the following kinds of problem arise in transmit operationsusing a linear array in the above kind of base station apparatus. Invector multiplication in the process of beam forming in a base stationapparatus, the signal received from a specific antenna is normally usedas a reference, and this signal is multiplied by a complex amplitude inwhich the real part is 1 and the imaginary part is 0. In expression (1)above, the complex amplitude multiplied for the first antenna has realpart 1 and imaginary part 0.

Therefore, in this vector multiplication, whereas the real part is 1 andthe imaginary part 0 for a beam in any direction for the first antenna,for the other antennas a component emerges that is cancelled inmultiplexing of beams from phase rotation. As a result, when this kindof vector multiplication is performed, the transmission power increasesonly for the first antenna, and with 50 users, for example, has thepower of 50 signals, and the dynamic range of the transmission amplifiermust be increased.

DISCLOSURE OF INVENTION

It is an objective of the present invention to provide a base stationapparatus and radio communication method that enable directivity formingto be performed in transmission using an array antenna that has aplurality of antenna elements, and that moreover enable the transmissionamplifier load to be alleviated.

A main subject of the present invention is to apply the same phaserotation to each element of a weight vector in a specific direction, todisperse the antenna elements giving the maximum or minimum amplitudeamong individual users, and decrease the amplitude bias toward aspecific antenna element, and thereby to alleviate the load on thetransmission amplifier.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the configuration of a conventionalbase station apparatus;

FIG. 2 is a drawing to explain beam forming;

FIG. 3 is a block diagram showing the configuration of a base stationapparatus according to Embodiment 1 of the present invention; and

FIG. 4 is a block diagram showing the configuration of a base stationapparatus according to Embodiment 2 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described specificallybelow with reference to accompanying drawings.

Embodiment 1

FIG. 3 is a block diagram showing the configuration of a base stationapparatus according to Embodiment 1 of the present invention. In thisbase station apparatus, it is assumed that a plurality of antennas 101-0to 101-n are arranged in a straight line at equal intervals. In FIG. 3,only the configuration of the transmitting side is shown; theconfiguration of the receiving side is the same as the conventionalconfiguration, and is therefore omitted.

In the base station apparatus shown in FIG. 3, signals received fromantennas 101-0 to 101-n are sent to RF sections 102-0 to 102-n providedfor the respective antennas, where the prescribed radio receptionprocessing (down-conversion, A/D conversion, etc.) is performed. Thesignal in the stipulated frequency band or intermediate frequency bandsubjected to this radio reception processing is the receive data onwhich demodulation processing is performed.

As regards transmit data, on the other hand, after digital modulationhas been performed for the respective users, linear addition isperformed by multiplexing sections 103-0 to 103-n, the prescribed radiotransmission processing (D/A conversion, up-conversion) is performed byRF sections 102-0 to 102-n, and the resulting signals are transmitted tothe respective users in a beam-formed state via antennas 101-0 to 101-n.This beam forming is performed for each user by processing sections104-0 to 104-n provided for each user.

Next, beam forming according to the present invention in processingsections 104-0 to 104-n will be described. Here, the case is describedin which the number of antennas n is 4 and the transmission weightcalculation method is the beam steering method.

These processing sections 104-0 to 104-n each comprise an arrivaldirection estimation circuit 1042 that estimates the direction ofarrival of a received signal, a transmission weight computation circuit1041 that finds the transmission weight based on the estimated directionof arrival, and a phase offset assignment circuit 1043 that assigns aphase offset to the transmission weight.

When beam forming (power distribution forming) according to thisembodiment is performed, the direction of arrival is first estimatedbased on the received signal in the arrival direction estimation circuit1042. The result of this estimation is sent to the transmission weightcomputation circuit 1041. Specifically, a plurality of beams are formedbeforehand by shifting the central angle, the communication quality ofthe signal received in each beam is found, and the beam with the bestcommunication quality is selected. Then, information on the angle ofthis selected beam is sent to the transmission weight computationcircuit 1041. Here, the SIR (Signal to Interference Ratio), receptionstrength, or the like, can be used as the communication quality of thesignal received in each beam.

In the transmission weight computation circuit 1041, the transmissionweight is calculated based on the angle information output from thearrival direction estimation circuit 1042. In the beam steering method,the weight in above expression (1) is used when communication isperformed with a mobile station in a specific direction (θ direction).

A phase offset is assigned to this transmission weight by a multiplier1044 for each antenna. This phase offset is prepared by the phase offsetassignment circuit 1043 so as to give a phase rotation of ψ. At thistime, the phase offset corresponding to each antenna is the same for oneuser. Thus, multiplying the transmission weight by the phase offset ismultiplying above expression (1) by expression (2) below. As a result,the transmission weight after phase offset multiplication is as shown inexpression (3) below.

Woffset=[exp(jψ),exp(jψ),exp(jψ),exp (jψ)]T  expression (2)

W′=[exp(jψ),exp(−j(π sin θ-ψ)),exp(−j(2π sin θ-104 )),exp(−j(3π sinθ-ψ))]T  expression (3)

If ψ in expressions (2) and (3) is set as ψ=n π sin θ (n=0, 1, 2, 3), inexpression (3) the complex amplitude for any antenna (the nth antenna)has a real part of 1 and imaginary part of 0. Therefore, by assigning inadvance the same phase offset stipulated by ψ=nπsin θ (n=0, 1, 2, 3) foreach antenna, for one user, it is possible to determine uniquely theantenna giving the maximum or minimum amplitude.

By changing this phase offset for each user, it is possible to changethe antenna giving the maximum or minimum amplitude for each user.Therefore, since it is possible to disperse antennas for which thetransmission power increases, it is possible to prevent concentration ofthe maximum amplitude on a specific antenna and the imposition of anexcessive load on the transmission amplifier. The phase offset betweenusers is controlled by an offset amount control circuit 106.

For the transmission weight to which a phase offset is assigned, thetransmission power is calculated by a multiplier 1045 and the actualtransmission power of each antenna is found. This actual transmissionpower per antenna is sent to a per-antenna power monitoring circuit 105.

The per-antenna power monitoring circuit 105 monitors the transmissionpower of each antenna determined for each user. Even if the phase offsetis changed for each user and multiplied by the transmission weight asdescribed above, there will still be cases where there are antennas witha high total transmission power and antennas with a low totaltransmission power in the overall base station apparatus. Theper-antenna power monitoring circuit 105 calculates the difference inthe transmission power of the antennas and prevents a large differencein transmission power from arising among the antennas. The detectionmethod can be implemented by determining a threshold value for thedifference in transmission power among the antennas, for example.

By this means, when directional transmission using user multiplexing isperformed in CDMA communication, it is possible to determine the phaseoffset for setting antenna balance adaptively, and to reliably dispersethe antenna elements giving the maximum or minimum amplitude amongindividual users. As a result, it is possible to decrease the amplitudebias toward a specific antenna element, and to further alleviate theload on the transmission amplifier.

If the difference in transmission power among the antennas is large inthe per-antenna power monitoring circuit 105, the per-antenna powermonitoring circuit 105 reports that fact to the offset amount controlcircuit 106. When a control signal is sent from the per-antenna powermonitoring circuit 105, the offset amount control circuit 106 changesthe phase offset to be assigned to the transmission weight of eachantenna and assigns it to the transmission weight.

There are no particular limitations on the method of changing the phaseoffset in the offset amount control circuit 106. For example, it ispossible for the offset amount control circuit 106 to send controlsignals to the phase offset assignment circuit 1043 of processingsections 104-0 to 104-n of all the users, and the phase offset to bechanged by the phase offset assignment circuit 1043, and it is equallypossible for the user with the highest transmission power and the userwith the lowest transmission power to be extracted by the per-antennapower monitoring circuit 105, and have the offset amounts of theextracted users exchanged. In this way, control is performed so as todisperse the antenna load.

By this means, the difference in total transmission power among theantennas is compensated, variance in transmission power among theantennas is made small, and the transmission amplifier load isalleviated. By means of such a method it is possible to adjust thedifference in transmission power among the antennas.

The transmission weight determined in this way is multiplied by thetransmit data by means of a multiplier 1046. Transmit data that has beenmultiplied by the transmission weight in this way is user-multiplexedfor each antenna by multiplexing sections 103-0 to 103-n.

Looked at from the viewpoint of a mobile station, the fact that a changein the propagation path conditions, and particularly phase rotation, hasbeen caused is recognized even when a phase offset is assigned by thebase station apparatus, and therefore phase compensation can beimplemented by normal processing. Consequently, there is no need for theaddition of special processing on the mobile station side whenreceiving.

Embodiment 2

FIG. 4 is a block diagram showing the configuration of a base stationapparatus according to Embodiment 2 of the present invention. The partsin FIG. 4 identical to those in FIG. 3 are assigned the same codes as inFIG. 3, and their detailed explanations are omitted. In this basestation apparatus, it is assumed that a plurality of antennas 101-0 to101-n are arranged in a straight line at equal intervals. In FIG. 4,only the configuration of the transmitting side is shown; theconfiguration of the receiving side is the same as the conventionalconfiguration, and is therefore omitted.

When beam forming according to this embodiment is performed, thedirection of arrival is first estimated based on the received signal inthe arrival direction estimation circuit 1042. The result of thisestimation is sent to the transmission weight computation circuit 1041.The procedure for estimating the direction of arrival is the same as forEmbodiment 1. In the transmission weight computation circuit 1041, thetransmission weight is calculated based on the angle information outputfrom the arrival direction estimation circuit 1042. In the beam steeringmethod, the weight in above expression (1) is used when communication isperformed with a mobile station in a specific direction (θ direction).Also, phase offset information for assigning ψ phase rotation is sent tothe arrival direction estimation circuit 1042 from the phase offsetassignment circuit 1043, and a phase offset is assigned to thetransmission weight. In other words, the product of above expression (1)and expression (2) is found by the transmission weight computationcircuit 1041. Therefore, although the phase offset itself is fixed foreach user, since it is multiplied by the transmission weight a phaserotation is applied randomly for each antenna. In this case also, it ispossible to determine uniquely the antenna giving the maximum or minimumamplitude.

By changing this phase offset for each user, it is possible to changethe antenna giving the maximum or minimum amplitude for each user.Therefore, since it is possible to disperse antennas for which thetransmission power increases, it is possible to prevent concentration ofthe maximum amplitude on a specific antenna and the imposition of anexcessive load on the transmission amplifier. The phase offset betweenusers is controlled by an offset amount control circuit 201.

The present invention is not limited to the above Embodiments 1 and 2,but can be implemented with various changes. For example, in aboveEmbodiments 1 and 2 the case is described where the number of antennasis 4, but the number of antennas can be other than 4.

Also, in above Embodiments 1 and 2 the case is described where thetransmission weight calculation method is beam steering, but anothermethod can be used. In this case, the same kind of control can beachieved even if, for example, null steering that lowers the gain to anextreme degree in a specific direction is used.

Moreover, in above Embodiments 1 and 2 the case is described where theobject of per-antenna power monitoring is the transmission weightmultiplied by the phase offset, but it is also possible to monitor thetransmission power and transmission weight of each user and incorporatethese two to control the phase offset.

In above Embodiments 1 and 2 the case is described where a linear arrayantenna in which a plurality of antenna elements are arranged in astraight line is used as an array antenna that has a plurality ofantenna elements, but the present invention is similarly applicable tocases where an array antenna in which a plurality of antenna elementsare arranged in a circle, an array antenna in which a plurality ofantenna elements are arranged two-dimensionally other than in a circle,or an array antenna in which a plurality of antenna elements arearranged three-dimensionally, is used as an array antenna that has aplurality of antenna elements.

A base station apparatus of the present invention employs aconfiguration comprising an array antenna composed of a plurality ofantenna elements, a transmission weight calculation section thatcalculates the transmission weight for each above-described antennaelement based on the direction of arrival of the received signal, aphase offset assignment section that assigns a user-specific phaseoffset to the above-described transmission weight, and a powerdistribution forming section that performs power distribution formingusing a transmission weight to which the above-described phase offsethas been assigned.

A base station apparatus of the present invention employs aconfiguration comprising an array antenna composed of a plurality ofantenna elements, a processing section that comprises for each user atransmission weight calculation section that calculates the transmissionweight for each above-described antenna element based on the directionof arrival of the received signal together with a phase offsetassignment section that assigns a user-specific phase offset to theabove-described transmission weight, and a directional transmissionsection that performs directional transmission by multiplexing, for eachabove-described antenna element, signals to which a transmission weight,to which the above-described phase offset has been assigned by theabove-described processing section, has been assigned.

According to these configurations, an identical phase offset is appliedto each antenna with regard to the weight vector in a specificdirection, and therefore it is possible to determine uniquely theantenna element giving the maximum or minimum amplitude. By this means,it is possible to disperse the antenna elements giving the maximum orminimum amplitude among individual users. As a result, it is possible todecrease the amplitude bias toward a specific antenna element, and toalleviate the load on the transmission amplifier.

A base station apparatus of the present invention, in theabove-described configuration, is equipped with a monitoring sectionthat monitors the transmission power for each antenna element, andemploys a configuration whereby the phase offset is changed when thedifference in transmission power among the antenna elements exceeds aprescribed value.

According to this configuration, it is possible to disperse reliably theantenna elements giving the maximum or minimum amplitude amongindividual users. As a result, it is possible to decrease the amplitudebias toward a specific antenna element, and to further alleviate theload on the transmission amplifier.

A base station apparatus of the present invention, in theabove-described configuration, employs a configuration whereby atransmission weight calculation section calculates the transmissionweight by means of beam steering, and the above-described phase offsetassignment section uses a phase offset such that ψ=n π sin θ (number ofantennas n=0, 1, 2, 3).

The power monitoring apparatus of the present invention employs aconfiguration comprising a processing section that comprises for eachuser a transmission weight calculation section that calculates thetransmission weight for each of a plurality of antenna elements based onthe direction of arrival of the received signal together with a phaseoffset assignment section that assigns a user-specific phase offset tothe above-described transmission weight, and a monitoring section thatchanges the phase offset when the difference in transmission power amongthe above-described antenna elements exceeds a prescribed value.

By this means, when directional transmission using user multiplexing isperformed in CDMA communication, it is possible to determine the phaseoffset for setting antenna balance adaptively.

A radio communication method of the present invention comprises atransmission weight calculation step of calculating the transmissionweight for each of a plurality of antenna elements comprising an arrayantenna, a phase offset assignment step of assigning a user-specificphase offset to the above-described transmission weight, a powerdistribution forming step of performing power distribution forming usingthe transmission weight to which the above-described phase offset hasbeen assigned, and a transmitting step of performing transmission withthe formed power distribution.

According to this method, an identical phase offset is applied to eachantenna with regard to the weight vector used for beam forming in aspecific direction, and therefore it is possible to determine uniquelythe antenna element giving the maximum or minimum amplitude. By thismeans, it is possible to disperse the antenna elements giving themaximum or minimum amplitude among individual users. As a result, it ispossible to decrease the amplitude bias toward a specific antennaelement, and to alleviate the load on the transmission amplifier.

A radio communication method of the present invention, in theabove-described method, comprises a monitoring step of monitoring thedifference in transmission power among the antenna elements, andchanging step of changing the phase offset when the above-describeddifference in transmission power exceeds a prescribed value.

According to this method, it is possible to disperse reliably theantenna elements giving the maximum or minimum amplitude amongindividual users. As a result, it is possible to decrease the amplitudebias toward a specific antenna element, and to further alleviate theload on the transmission amplifier.

The power monitoring method of the present invention comprises a step ofcalculating the transmission weight for each of a plurality of antennaelements based on the direction of arrival of the received signal, aphase offset step of assigning a user-specific phase offset to theabove-described transmission weight, and a step of monitoring thetransmission power for each of the above-described plurality of antennaelements and changing the phase offset when the difference intransmission power among the above-described antenna elements exceeds aprescribed value.

According to this method, when directional transmission using usermultiplexing is performed in CDMA communication, it is possible todetermine the phase offset for setting antenna balance adaptively.

According to the present invention, as described above, an identicalphase rotation is applied to each element of the weight vector in aspecific direction, the antenna elements giving the maximum or minimumamplitude are dispersed among individual users, and thye amplitude biastoward a specific antenna element is decreased, so that, in beam formingby means of an array antenna, it is possible to disperse thetransmission amplitude load that arises when a plurality of beams aremultiplexed.

When directional transmission using user multiplexing is performed inCDMA communication, it is possible to determine the phase offset forsetting antenna balance adaptively, and to disperse reliably the antennaelements giving the maximum or minimum amplitude among individual users.As a result, it is possible to decrease the amplitude bias toward aspecific antenna element, and to further alleviate the load on thetransmission amplifier. Consequently, it is no longer necessary to use alarge amplifier dynamic range when designing a base station, and it ispossible to reduce the scale and cost of the apparatus.

This application is based on the Japanese Patent Application No.HEI11-276223 filed on Sep. 29, 1999, entire content of which isexpressly incorporated by reference herein.

Industrial Applicability

The present invention is applicable to a base station apparatus in adigital radio communication system.

What is claimed is:
 1. A base station apparatus comprising: an arrayantenna comprising a plurality of antenna elements; an arrival directionestimation circuit that estimates a direction of arrival of a receivedsignal; a transmission weight calculating section that calculates atransmission weight for each of said antenna elements based on thedirection of arrival of the received signal; an offset amount controlcircuit for controlling a phase offset to be assigned to thetransmission weight of said each of said antenna elements; a phaseoffset assigning section that receives said phase offset and assigns auser-specific phase offset to the transmission weight of said each ofsaid antenna elements; a directional transmitting section that performsdirectional transmission using the transmission weight of said each ofsaid antenna elements to which the user-specific phase offset has beenassigned; and a monitoring section that monitors a transmission powerfor each of said antenna elements, wherein the user-specific phaseoffset is changed when a difference in transmission power among saidantenna elements exceeds a prescribed value.
 2. The base stationapparatus according to claim 1, wherein the transmission weightcalculating section calculates the transmission weight of said each ofsaid antenna elements by means of beam steering and said phase offsetassigning section assigns the user-specific phase offset such that phaserotation ψ=nΠ sin θ, where a number of antennas n equals 0, 1, 2 or 3and θ represents a direction of a mobile station to the base stationapparatus.
 3. A base station apparatus comprising: (a) an array antennacomprising a plurality of antenna elements; (b) a processing sectionthat comprises for each user of a plurality of users: (i) an arrivaldirection estimation circuit that estimates a direction of arrival of areceived signal; (ii) a transmission weight calculating section thatcalculates a transmission weight for each of said antenna elements basedon the direction of arrival of the received signal; (iii) a phase offsetcontrol section that controls a phase offset to be assigned to thetransmission weight of each of said antenna elements; and (iv) a phaseoffset assigning section that receives said phase offset and assigns auser-specific phase offset to said transmission weight of each of saidantenna elements, wherein said processing section produces signalscorresponding to each of said antenna elements, said signals having saidtransmission weight and said user-specific phase offset assignedthereto; and (c) a directional transmitting section that performsdirectional transmission by multiplexing, for each of said antennaelements, said signals that correspond to each of said users and thathave said transmission weight and said user-specific phase offsetassigned thereto.
 4. A power monitoring apparatus comprising: aprocessing section that comprises a transmission weight calculatingsection that calculates a transmission weight for each of a plurality ofantenna elements, based on a direction of arrival of a received signal,and a phase offset assigning section that assigns a user-specific phaseoffset to the transmission weight for each of said antenna elements; anda monitoring section that monitors a transmission power for each of saidantenna elements and changes the user-specific phase offset when adifference in transmission power among said antenna elements exceeds aprescribed value.
 5. A radio communication method comprising the stepsof: estimating a direction of arrival of a received signal; calculatinga transmission weight for each of a plurality of antenna elementsconstituting an array antenna; assigning a user-specific phase offset tosaid transmission weight for each of said antenna elements; forming apower distribution for said antenna elements using the transmissionweight for each of said antenna elements to which said user-specificphase offset has been assigned; transmitting said power distributionthrough said antenna elements; monitoring a difference in transmissionpower among said antenna elements; and changing the user-specific phaseoffset when said difference in transmission power exceeds a prescribedvalue.
 6. A power monitoring method comprising the steps of: calculatinga transmission weight for each of a plurality of antenna elements basedon a direction of arrival of a received signal; assigning auser-specific phase offset to said transmission weight of each of saidantenna elements; and monitoring a transmission power for each of saidantenna elements and changing the user-specific phase offset when adifference in transmission power among said antenna elements exceeds aprescribed value.